1. Field of the Invention
The present invention relates to a method and apparatus for performing alignment by photoelectrically detecting the alignment mark formed on a semiconductor wafer, a plate for liquid crystal display, or the like.
2. Related Background Art
Traditionally, there has been generally used in positioning the wafer, plate, or the like (in alignment), a method of photoelectrically detecting the alignment mark formed at a predetermined position on a substrate through the objective lens of a microscope.
The photoelectric detection method is roughly divided into two kinds, i.e., the light beam scanning method in which the mark is relatively scanned by the spot of a laser beam or the like, and the scattering rays of light or refraction light generated by the mark is received by a photomultiplier, photodiode, or the like; and the method which utilizes image signals obtained by a television camera (Vidicon tube or CCD) picking up the enlarged image of a mark evenly illuminated.
In either case, the waveform of such photosignal is processed to obtain the central position of the mark.
Although the light beam method and the pick-up method are completely different in its structures of scanning system, these two are considered here as an electrical-optical scanner, respectively (hereinafter referred to as E.O.S.).
Among such E.O.S.'s, there is known a technique as a method of detecting the mark position by carrying the wafer stage one-dimensionally against the laser beam spot such as disclosed in U.S. Pat. No. 4,655,598, U.S. Pat. No. 4,677,301, and U.S. Pat. No. 4,702,606.
Also, there is known a technique as a method of detecting the mark position within the region of the one-dimensional scanning subsequent to the positioning of the wafer stage by a designed value such as disclosed in U.S. Pat. No. 4,390,279, and U.S. Pat. No. 4,566,795.
Also, as an E.O.S. using a pick-up method, there is known a technique such as disclosed in U.S. Pat. No. 4,402,596, U.S. Pat. No. 4,679,942, and U.S. Pat. No. 4,860,374.
In these conventional techniques, a monochromatic light is used as a scanning beam or mark illuminating light mainly for two reasons given below.
1. In a projection type aligner (stepper), a single-wavelength illuminating light or laser beam is used in order to avoid any large chromic aberration for the type which detects the wafer mark through the projecting optical system.
2. A monochromatic laser beam is used to enable a fine spot convergence of the beam for performing a high-luminance and high-resolution detection. When a monochromatic illuminating light (or beam) is used as set forth above, the obtainable S/N ratio is comparatively large. However, there appears an interference phenomenon due to the monochromaticity because a photoresist layer of 0.5 .mu.m-2 .mu.m thick is usually formed all over the wafer surface, and this often results in a detection error when the mark position is detected or makes an objective image unclear.
Therefore, in order to reduce the interference phenomenon caused by the resist, there has been proposed in recent years the application of multi-wavelength or wide band to the illuminating light.
For example, an illuminating light is produced by a halogen lamp or the like for the pick-up type E.O.S., and if the wavelength bandwidth thereof is set for approximately 300 nm (with the exception of the photosensitive region for the resist), the coherence of the rays themselves reflected from the resist surface and the wafer surface almost disappears; thus making it possible to carry out the detection on a clear image. Therefore, in the pick-up method, if only a white (wide band) illuminating light is used with an achromatic image-formation optical system, an extremely precise alignment sensor which is not affected by the resist is obtainable.
As the above describes, with the application of a polychromatic or white illuminating light, it becomes possible to restrict the generation of the interference fringe for an excellent image detection. Then, the extremely small factors causing errors which have passed unnoticed come to the fore.
In other words, since the staged structure of the alignment mark is clearly detected, a slight difference in the profiles of the mark edges becomes capable of affecting the precision of the detection or alignment.
Traditionally, various algorithms have been worked out for image signal processings. However, none of them have ever taken into consideration such slight changes in the mark edge profiles, and there has automatically been a limit for an overall improvement of the alignment accuracy.